Re: [PATCH 4/6] pidns: Don't allow new pids after the namespace is dead.

[Oleg Nesterov <oleg@redhat.com>]

On 06/20, Eric W. Biederman wrote:
>
> In the case of unsharing or joining a pid namespace, it becomes
> possible to attempt to allocate a pid after zap_pid_namespace has
> killed everything in the namespace.  Close the hole for now by simply
> not allowing any of those pid allocations to succeed.  At least for
> now it is too strange to think about.

Well, I didn't read the next patch, so I don't understand the changelog.

Still, a couple of stupid questions. Feel free to ignore me...

> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -20,6 +20,7 @@ struct pid_namespace {
>  	struct kref kref;
>  	struct pidmap pidmap[PIDMAP_ENTRIES];
>  	int last_pid;
> +	atomic_t dead;

Why it is atomic_t? It is used like a simple boolean, and the next
patch doesn't use ns->dead.

> @@ -248,6 +248,10 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>  	struct pid_namespace *tmp;
>  	struct upid *upid;
>
> +	pid = NULL;
> +	if (atomic_read(&ns->dead))
> +		goto out;
> +
> [...snip...]
> @@ -161,6 +162,7 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
>  	 *
>  	 */
>  	read_lock(&tasklist_lock);
> +	atomic_set(&pid_ns->dead, 1);
>  	nr = next_pidmap(pid_ns, 1);

The only caller of alloc_pid() is copy_process(). So, at first glance this
patch tries to block the attempts to create the tasks in this namespace.

But what if copy_process() has already called alloc_pid() using this ns,
but didn't do attach_pid() yet?

Oleg.